<p>Phytoplankton abundance is a fundamental parameter for understanding the water quality and trophic levels of lakes and ponds. However, the fluorometers used for quantification are expensive. In this study, we fabricated a low-cost fluorometer to analyse in vivo fluorescence of phytoplankton fluorescence using commercially available electric parts. To evaluate the functionality of the fluorometer, we compared the in vivo fluorescence intensity to chlorophyll <i>a</i> concentrations determined by the extraction method for 22 lake water samples. The fluorescence values measured using our apparatus were significantly correlated with the extracted chlorophyll <i>a</i> concentrations (<i>r</i>^<i>2</i> = 0.935), displaying a linear relationship up to at least 366.2&#xa0;µg L⁻¹. Therefore, it is demonstrated that this apparatus has sufficient sensitivity to evaluate the phytoplankton abundance in oligo-to eutrophic lakes and ponds.</p>